Three trials involving 251 crossbred pigs were conducted to establish response surfaces for effects of Ca/P levels (70, 85, 100, 115 and 130% of NRC [1979] estimated requirements) and time (weaning to market) on bone criteria and to compare bending and shear bone testing. Nine replicates, each consisting of five pigs, were used over the three trials. One pig per replicate was slaughtered every 4 wk following the start of the trials. Third metacarpals and metatarsals (3M) and fourth metacarpals and metatarsals (4M) were collected at slaughter; the bone length, bending or shear stress, and dry, fat-free ash percentage (DFF%) were determined. Most bone criteria increased (P less than .01) at a decreasing rate (P less than .05) in response to increasing dietary Ca/P intake and increased linearly (P less than .01) with time on test, although bone length and stress increased at a decreasing rate with time on test. Asymptotic models relating continuous effects of total Ca + P intake ratio (CAP; expressed as a percentage of NRC) and number of days on test before slaughter to bone measurements were fit to least squares slaughter time by diet means. Bone length and DFF% reached 98% maximum at or near 100% the NRC estimated CAP level; however, to reach 98% of maximum bone shear and breaking stress higher CAP levels were required. Shear testing of bones was more desirable than bend testing as a method of determining bone strength due to reduced variability, better discrimination of Ca/P and time effects and ease of calculation.

Tibia biopsies were taken from 75 live pigs at 4-wk intervals and from 251 slaughtered pigs to evaluate bone biopsy as a procedure for determining Ca/P status in pigs fed 70, 85, 100, 115 and 130% of the NRC (1979) estimated dietary Ca and P percentage requirements from weaning to market. Least squares means and SE of live and slaughter biopsy wet weight, ash weight and dry, fat-free ash percentage (DFF%) were compared at each time in each trial and found not to differ. Diet and time effects on ash weight, ash percentage of wet weight and DFF% of the biopsy core also did not differ greatly between slaughter and live biopsies and generally responded linearly and quadratically (P less than .01) to increasing Ca/P level and time. Biopsy measures were correlated (P less than .05) with third and fourth metacarpal and metatarsal length, bending and shear stress and DFF%. Means for slaughter biopsy DFF% did not differ greatly from the average of third and fourth metacarpal and metatarsal DFF% from slaughter pigs. Means for live and slaughter biopsy DFF% were lower than those for whole bones for the 70 and 85% NRC estimated Ca/P levels, but not for the 100, 115 and 130% NRC levels. Bone biopsy offers potential as a reliable noninvasive procedure for monitoring Ca/P status of swine from weaning to market, but it needs further study for use in Ca/P research in swine.

Three trials using 300 pigs were conducted to establish response surfaces for effects of calcium and phosphorus (Ca/P) levels and of time on postweaning pig performance. Five dietary Ca/P levels (70, 85, 100, 115 and 130% of the NRC [1979] estimated dietary Ca and P requirements) were fed from weaning to market weight. Twelve replicates, each consisting of five pigs, were used across the three trials, and one pig per replicate was slaughtered every 4 wk following the start of the trials. Efficiency of feed utilization (G:F) was insensitive to diet within the range of Ca/P levels used, whereas ADG and average daily feed intake (ADFI) were linearly (P less than .01) and quadratically (p less than .01) related to both diet and weigh period. Asymptotic models relating continuous effects of total Ca + P intake (average grams per day of Ca + P) relative to the NRC estimated Ca + P intake and days on test at the midpoint of each weigh period were fitted to the data and used to derive response surfaces for ADG and ADFI. Ninety, ninety-five and ninety-eight percent of maximum ADG, ADFI and G:F also were determined; 98% of maximum ADG was reached with a Ca and P intake of 99.8% of NRC Ca/P estimates, whereas Ca and P intakes of 95.3 and 83.5% of NRC estimates were required to produce 98% of maximum ADFI and G:F, respectively.

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